Automatic control of multi-speed vehicle transmissions is well-known. Speed ratio changes are effected automatically, usually by reference to vehicle speed and accelerator pedal position. Thus typically an upshift will be delayed when accelerator pedal position is indicative of a demand for rapid acceleration, allowing engine speed to be greater at the shift point than for the case of a demand for lesser acceleration.
Electronic control techniques permit automatic speed ratio change to be initiated according to an electronic shift map whereby road speed, accelerator pedal position and other factors are provided as inputs to an electronic processor, and algorithms and/or look-up tables allow the instant shift point to be determined.
Many control systems for automatic transmissions include gradient recognition, whereby the shift point may vary according to the instant road gradient. Thus when travelling uphill, a lower speed ratio may remain engaged due to the raising of a shift point with respect to engine speed. Conversely a shift point may be at a reduced engine speed when on a downhill gradient. In practice a plurality of shift maps corresponding to step changes of gradient may be provided. These arrangements may be used to improve fuel efficiency of a vehicle engine as compared with a shift map which is insensitive to gradient, and may also better meet driver expectations.
One method of a gradient determination compares instant engine torque against acceleration (rate of change of speed), and provides an algorithm to calculate gradient by reference to deviation from base data obtained from running a vehicle on a level road in benign conditions. Other factors which may be taken into account include vehicle mass, altitude and ambient temperature.
If the vehicle accelerates faster than indicated by base data, it may be assumed that the vehicle is on a down gradient, and if slower is on an up gradient. Likewise a level gradient can be detected after an up or down gradient has ceased. Suitable thresholds are chosen appropriate to predetermined gradient steps. This measure may also be used to compensate for changes in acceleration due to high vehicle mass or due to a towed trailer.
In place of engine torque, the comparison may use tractive effort or some other measure which relates to the motive force of the vehicle.
One characteristic of sensing road gradient by comparing engine torque and rate of change of speed is that adoption of an alternative shift map requires that sufficient distance be travelled on the gradient for a reliable determination to be made; thus adoption of an alternative shift map may not occur until the gradient has been substantially entered.
A further characteristic is that the start and end of a gradient is generally gradual. Accordingly the comparison of engine torque and rate of change of speed may not be assumed to give a reliable indication of gradient until the gradient has become consistent. In some circumstances the transmission may be caused to select sequential shift maps as the gradient progressively changes.
The effect of these characteristics is that the most suitable shift map may not be adopted as early as is desirable, and/or the vehicle is perceived to exhibit to the driver a variability associated with different shift points, which may be assumed to indicate a fault when no fault is present.
An apparatus and method of overcoming these effects would be desirable.
WO-2011/138097 discloses a recognition system of road signs whereby an imminent change of gradient is identified, and in consequence the shift points of an automatic vehicle transmission are changed. Whilst this system may be useful, it relies upon consistent position of road signs with respect to the commencement of the gradient, for otherwise a change of shift map may occur too early or too late. Such a system does not address cessation of a gradient, which would also require to be signed.